This invention relates generally to a control circuit for a three-phase AC motor and particularly to a solid state control system, comprising primarily digital logic circuitry, for controlling the operation of the three-phase submersible pump motor while at the same time protecting it against adverse conditions.
In pumping fluid from an oil well, one conventional practice is to position a submersible three-phase electric motor and pump assembly in the well bore hole near the bottom thereof. A three-phase power supply of appropriate voltage (for example, 1500 volts per phase) couples to and drives the pump motor, which may have a rating of well over 200 horsepower. Since the average depth of an oil well is usually around 9,000 feet, severe heat and pressure may exist in the environment where the motor operates. It is important to shut the pump motor down in the presence of those conditions to prevent it from overheating. The motor should also be protected against overload or short circuit conditions which would pass more current through the motor than it could tolerate. Protection should also be provided against an underload or undercurrent condition, such as would occur when there is no fluid in the oil well, a condition known as "pump-off". In addition to the motor shutdowns occasioned by undesirable conditions or environmental changes, it is usually desirable to turn the motor off automatically in response to certain physical conditions, such as when a retaining tank is filled up.
Preferably, motor shutdown is delayed for a prescribed period to avoid unnecessary nuisance shutdowns resulting from transient effects or momentary conditions. Moreover, after the pump motor is de-energized in response to an underload condition caused by pump-off, it is desirable that the motor automatically restart after a time period sufficiently long to allow fluid to flow into the well.
Control systems have been developed in the past for accomplishing the above result. Unfortunately however, they are of relatively complex and costly construction, requiring considerable space and consuming substantial power. Furthermore, the previously developed systems have poor reliability and are inefficient in operation. In sharp contrast the present invention performs not only all of the functions of the prior systems but additional functions, and this is achieved by means of a low-cost, low-power, highly efficient and reliable control system that can be contained within a very small fraction of the space needed by the prior systems.